Gastrulation and Neurulation Flashcards
What is the form of the disk at the end of week 2?
Bi-laminar disc (epiblast and hypoblast)
What happens at the beginning of week 3?
Gastrulation begins
What are the key structures on the bi-laminar disc as gastrulation begins?
+ Anterior visceral endoderm of hypoblast defines anterior
+ Primitive streak (groove on epiblast)
+ Primitive pit and associated primitive node
What happens at the streak and node during gastrulation when the endoderm is formed?
Epiblast cells stream into the embryo along the primitive streak/node and replace hypoblast cells
What happens during gastrulation when the mesoderm is being formed?
+ Epiblast cells stream into the embryo along the primitive streak between the two layers
+ In front of the node a specialised mesoderm structure called the notochord forms between the two layers
What is the notochord?
+ An important signalling structure
+ Primitive beginning to the spine
+ Appears in embryos as a small flexible rod made from mesoderm cells
What is neurulation?
+ The folding process in vertebrate embryos, - transformation of the neural plate into the neural tube
+ The embryo at this stage is termed the neurula
What does the primitive node/notochord organise?
Dorsal structures
Why is the notochord an important signalling structure?
It releases factors (chordin, noggin, follistatin) which block the effects of BMP, allowing neural tissue to form instead of skin
What happens to the neural plate at around 19 days?
+ Neurulation induced by notochord (deep to neural epithelium)
+ Epithelial cells become columnar
+ The plate will form a tube (neurulation)
+ Day 19+ a midline groove becomes apparent
What happens around 20-21 days (end of third week)?
+ Cells on plate edge thicken and form neural folds and a neural groove
What happens during day 22+ of the fourth week?
There is fusion in the cervical region of the neural folds and neural tube beings to form
What causes the curvature of the columnar cells to form the tube?
Actin rings around apex of columnar cells constricts
Approx. how long does it take for the rostral/nose-end to close?
25 days
What forms at the rostral/nose-end?
rostral neuropore
Approx. how long does it take for the caudal/tail-end to close?
27 days
What forms at the caudal/tail-end?
Caudal neuropore
What happens if the rostral neuropore fails to close?
Anencephaly
- brain fails to develop
What happens if the caudal neuropore fails to close?
Some forms of spina bifida
- neural tissue remain open to the environment
Name cells which are derived from neural crest cells
\+ Cranial nerve ganglia \+ Dorsal root ganglia \+ Autonomin ganglia \+ Adrenal medulla \+ Schwann cells
+ Peripheral glial
+ Smooth muscle of cardiac flow
+ Odontoblasts
+ Craniofacial skeleton
+ Thyroid parafollicular (C) cells
+ Melanocytes
…
What are two possible syndromes that can develop due to defective neural crest development?
+ Waardenburg’s Syndrome (1/50,000)
+ Treacher Collins Syndrome (autosomal dominant 1/50,000)
What are features of Waardenburg’s Syndrome?
+ Some types have Pax-3 deletion
+ Pigment abnormalities (even albinism)
+ Deafness
+ Heterochromia of eyes
+ Telecanthus (widely serparated eye “corners”)
+ Other types show additional constipation (SOX10)
What are features of Treacher Collins Syndrome?
+ Defective protein called Treacle (TCOF1 gene)
+ Failure of formation/apoptosis of neural crest cells
+ Abnormal eye shape
+ Micrognathia
+ Conductive heating loss
Underdeveloped zygoma
+ Malformed ears
What are the different layers of mesoderm?
+ Paraxial
+ Intermediate
+ Lateral plate
What is formed when the paraxial mesoderm becomes segmented?
Somites
What are the features of somite formation?
+ Cells have a timer along paraxial mesoderm
+ Go through cycles every 90 mins defined by notch signalling clock
+ Wave of FGF signal that passes along the embryo
+ When wave passes cells they are programmed to change into part of a somite
What are the crucial timings of the somite formation cycle?
+ If the wave passes cells early in the cycle they become the front end of the segment
+ If the wave passes cells late in the cycle they become the tail end of the segment
The process is repeated over and over
What do homeobox genes do?
+ Direct formation of body structures e.g from body segments
+ Critical for forming body axes
What are derivates of the paraxial mesoderm?
+ Head
+ Somites (sclerotome, myotome, dermatome)
What are derivates of the intermediate mesoderm?
+ Kidneys
+ Gonads
What are derivates of the lateral plate mesoderm?
+ Splanchnic (coverings, viscera, CVS)
+ Somatic (parietal, body wall)
What are derivates of the midline mesoderm?
+ Prechordal plate
+ Notochord (nucleus pulposus)
What does the yolk sac become?
The gut tube (remnant of gut tube)
How does the gut tube form?
Amniotic cavity enwraps and surrounds the embryo, almost completely cutting off gut from the yolk sac
What does the urinary bladder have a connection with?
The cloaca
When does the vitelline duct usually obliterate?
Around 5-6 weeks
What is and what causes Meckels diverticulum?
+ A congenital abnormality of the small intestine
+ Due to remnant of vitalline duct (yolk sac/gut connection)
What are some derivatives of the ectoderm?
\+ Epidermis \+ Lens and retina \+ Olfactory epithelium \+ Oral cavity epithelium \+ Sensory organs of ear \+ Glands: salivary, sweat, mammary \+ Adenohypophysis
